In recent years, the reduction in size and weight for cellular phones, laptop computers, and the like has progressed rapidly, and batteries as power sources for driving the phones, the computers, and the like have been required to have higher capacities. Further, under such circumstances, lithium ion secondary batteries have been widely used as power sources.
Further, as a positive electrode for use in such a lithium ion secondary battery, a positive electrode has been proposed that has a positive electrode combination material including a positive electrode active material that produces a potential of 4.5 V or higher on the basis of metal lithium, a conduction aid, and a binder, wherein the conduction aid has non-graphitizable carbon and carbon black, and a ratio (SC/SA) of a surface area SC of the conduction aid in the positive electrode combination material to a surface area SA of the positive electrode active material therein is 0.5 or more and 2.5 or less; and a lithium ion secondary battery using the positive electrode (see Patent Document 1).
Further, the electrode configured as mentioned above is supposed to be capable of suppressing, in the positive electrode active material that produces a potential of 4.5 V or higher, a decrease in coulombic efficiency due to oxidative decomposition of a solvent in a non-aqueous electrolytic solution, cell bulge due to gas generation therein, and cycle characteristic degradation due to expansion and contraction of graphite as a conduction aid.
However, when non-graphitizable carbon that is lower in electron conductivity than carbon black and graphite is used as the conduction aid, degradation is caused in the charge/discharge rate characteristics of the positive electrode.
Therefore, the positive electrode for a lithium ion secondary battery using the positive electrode active material that produces a potential of 4.5 V or higher has a problem that it is difficult to achieve a balance between great charge/discharge rate characteristics and inhibited oxidative decomposition of the solvent in the non-aqueous electrolytic solution.
Patent Document 1: Japanese Patent Application Laid-Open No. 2011-129442